1. Field of the Invention
The present invention relates to a separation method for zinc sulfide, in more detail, relates a separation method for zinc sulfide, in hydrometallurgy by a High Pressure Acid Leach for nickel oxide ore comprising a leaching and solid/liquid separation step, a neutralization step, a zinc removal step, and a nickel recovery step, which can improve filtration performance of zinc sulfide to be formed and inhibit decrease of nickel recovery ratio, in the zinc removal step in which zinc sulfide is formed by adding a sulfurizing agent to the neutralization final liquid containing zinc as well as nickel and cobalt and then said zinc sulfide is separated to obtain a mother liquid for nickel recovery containing nickel and cobalt.
2. Description of the Prior Art
In recent years, a High Pressure Acid Leach using sulfuric acid has been attracting attention as hydrometallurgy for nickel oxide ore. This process is advantageous in energy and cost because the process consists of a consistent hydrometallurgical step without including a dry step such as reduction and drying process, differing from a pyrometallurgy which is a conventional general refining process for nickel oxide ore, and further the process has an advantage that a sulfide containing nickel and cobalt having an increased nickel content up to around 50% by mass (hereinafter, may be referred to as nickel cobalt mixed sulfide) can be obtained.
The above-described High Pressure Acid Leach process comprises, for example, the following steps:
(a) a leaching and solid/liquid separation step in which sulfuric acid is added to a slurry of nickel oxide ore, leaching is conducted under a high temperature and pressure, then the residue is separated while the leaching slurry is subjected to a multistage washing, to obtain a leach liquor containing impurity elements as well as nickel and cobalt;
(b) a neutralization step in which pH of the above leach liquor is adjusted and neutralized precipitate containing impurity elements is separated, to obtain a neutralization final liquid containing zinc as well as nickel and cobalt.
(c) a zinc removal step in which zinc sulfide is formed by adding a hydrogen sulfide gas to the above neutralization final liquid and said zinc sulfide is separated, to obtain a mother liquid for nickel recovery containing nickel and cobalt; and
(d) a nickel recovery step in which mixed sulfide containing nickel and cobalt are formed by adding a hydrogen sulfide gas to the above mother liquid for nickel recovery, and said mixed sulfide are separated.
Here, in the neutralization step, for example, the leach liquor obtained from the above leaching and solid/liquid separation step is introduced into a neutralization tank, neutralized by adding a calcium carbonate slurry, and the resultant precipitation of hydroxides is separated by a solid/liquid separation, to obtain a neutralized precipitate and a neutralization final liquid.
In addition, in the zinc removal step, the above neutralization final liquid is introduced into a sulfurization reaction tank, a sulfurizing agent such as hydrogen sulfide gas, sodium hydrosulfide is added thereto to sulfurize zinc, copper, etc. contained in said neutralization final liquid, after that, a solid/liquid separation is conducted using a filter press etc., to obtain zinc sulfide and a mother liquid for nickel recovery (see, for example, Patent Literatures 1 and 2) containing nickel an cobalt. Incidentally, since the mixed sulfide obtained in the above-described High Pressure Acid Leach are further used as a raw material to refine up to electrolytic nickel and electrolytic cobalt, in the zinc removal step, a Zn concentration in the final liquid is required to be decreased to 0.001 g/L or less.
Meanwhile, when the method to separate zinc as a zinc sulfide as described above is employed in a practical plant of the hydrometallurgy for nickel oxide ore by the above High Pressure Acid Leach, since clogging of filter cloth occurs in the filtration and separation of the sulfide formed using a filter cloth, washing operation and replacement operation of a filter cloth had been carried out to improve filtration rate. Moreover, a frequency of washing operation or replacement operation of the filter cloth had been widely fluctuated from around once per several days to around once per several hours. Namely, generally in the sulfurization reaction using hydrogen sulfide of sulfuric acid aqueous solution containing zinc as well as nickel and cobalt, due to the reaction in which sulfide precipitate consisting of fine particles is formed, the sulfide formed covers the surfaces of controlling instruments such as electrode to make an accurate measurement of pH difficult, furthermore, due to occurrence of clogging of a filter cloth in filtration and separation using the filter cloth of the sulfide formed, filtration rate gradually decreases and finally does not proceed. For this reason, when washing operation or replacement operation of the filter cloth to improve the filtration rate is frequent, it had become a factor which greatly impairs an operational efficiency. Further, the sulfide having such a poor filtration performance had been accompanied by nickel and cobalt together with adhered water, and discharged out of the system to be lost.
In order to solve this problem, generally known improvement measures such as addition of a flocculant to agglomerate the sulfide precipitate consisting of fine particles etc. were tried, but no effective result was obtained in any case.
In view of the above circumstance, in a method in which zinc sulfide is formed by adding a hydrogen sulfide gas to a sulfuric acid aqueous solution containing nickel, cobalt and zinc and said zinc sulfide is separated, a separation method for zinc sulfide which can improve the filtration performance of zinc sulfide formed has been demanded.
[Patent Literature 1] JP-A-06-116660 (Pages 1 and 2)
[Patent Literature-2] JP-A-2005-350766 (Pages 1 and 2)